Time division multiplex communication systems



Ot. 27,v 1959 1 R. F. HARRIS 2,910,542

TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS 794,5( f BY TORNEY Oct. 27,l 1959 L. R. F. HARRIS TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS Filed Nov. 24, 1954 1l Sheets-Sheet 2 ATTORN EY oct. 27, 1959 L. R. F@ HARRIS 2,910,542

TIME DIvIsION MULTIPLEX COMMUNICATION SYSTEMS Filed Nov. 24, 1954 I 11 sheets-sheet 3 Flc-1.414.

INVENTQR L/'a/le/ HTF/340073.'

oct. 27, 1959 L. R. R; HARRIS 1 2,910,542v TME DIVISILON MULTIPLEX COMMUNICATION SYSTEMS .1 Filed Nov. 24, 1954 11 sheets-sheet 4 RM1 m E MODI/A701115 K1 PHL3 PSL PHS 44 Pin 5709065 afl/IFE Y ATTORNEY Oct. 27, 1959 L.. R. F. HARRIS 2,910,542

TIME DIVISION MULIIPLEX COMMUNICATION sYsIEMs Filed Nov. 24, 1954 I. 11 sheets-sheet 5 sat-crans BY MMV/y@ ATTORNEY Oct. 27, 1959 L. R. F. HARRIS 2,910,542

TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS Filed Nov. 24, 1954 l1 Sheets-Sheet 6 L DCM ps1 m Pn PM PL pn. pu. cu. I6 |718 7 6 9 14 8% 6 a I4 I I I I-ncma sn 6' Mx7 nous antena/i z lggg; DCiL 15 DCIL5 DCILIG PSL PILE

DclL? DCILB SEL .STIECTO/RS' PIL 15 PL I8 DILQ SEL 7 RFU ATTORN EV Oct. Z7, 1959 V l.. R. F. HARRIS 2,910,542

TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS Filed Nov. 24. `1954 11 sheets-'sheet 7 Fre. 6A.

DCIL Y. cs 13.2" C'f Q2 Q3 pus er1-oen p534 PLMS P6625 onf/cf STORHGE DE l//C E PEG/5 767? NVENTOR ATTORNEY Oct. 27, 1959 L. R. F. HARRIS 2,910,542

TIME DIVISION MOLIIPLEX COMMUNICATION sYsIEMs Filed Nov. 24, 1954 11 Sheets-Sheet 8 nog M7 F'I l""| r M M13 .l POLB 14 s' g1g LJ I. v-

PILIS RCD?! RCDTI pw?" (53%???- P611 J m11 DCIL" I-.J .-l

nena 2 r ses? :L Er nmz PCG" I REG' I' @f5/5TH? Oct. 27, 1959 L.. R. F. HARRIS 2,910,542

TIME DIVISION MULIIPLEX COMMUNICATION sYsIEMs Filed Nov. 24, 1954 I1 sheets-sheet 9 STORAGE' DEV/C' ATTORNEY Oct. 27, 1959 L.. R. F. HARRIS 2,910,542

TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS Filed NOV. 24, 1954 11 Sheets-Sheet l0 ATTORNEY Oct. 27, 1959 L. R. F. HARRIS TIME DIVISION MULTIPLEX COMMUNICATION SYSTEMS Filed Nov. 24, 1954 l1 Sheets-Sheet l1 LlONEL R. F. HARRIS ATTORNEY m v 1N. 288228 22.6.82 225.82 2...... f 322.582 m5522221 l m2... m FL FL IL FL M 2.325 2.825 J/ 2.m|\\\ )M ...2.2m` 2.5.28 3 l 3120228 2.55.22 2. :82.0 222 @22 w $8 @2....3 2:2. ....25 .205% .m2952228 2.2 2.22.2.. .0.225282 22.82222. f 1|.. 1|. 1A.. 22m mmlm .0.225282 2.25282 l52.22.2222. Esl .J .www 2....... ...2.... 2.62 2022....0 22:..8 ....55 8 1 fol@ m u .22252 n SQ Avv m2... A 22252 m @MIT m2... 25m .0.222.532 m5@ m2953232 E.. S2222. ...m5 2285282 23 @252.222 2o.w$mm...=.m ammm @252.222

United States Patent O TIME DlvlsIoN `MULTIPI-.Ex CoimuICATIo SYSTEMS Lionel Roy Frank Harris, Kenton, England, assignr to Her Majestys Postmaster General, London, YEngland Application November 24, 1954, Serial No. 471,072 17 Claims. rei. 1179-15) 'd tions between circuits and transmitting speech.

In this specification an electric pulse is defined as a sudden change of current or'voltage'lasting` for a brief period. A succession of pulses equally spaced in time constitutes a pulse train. A group of pulse trains are said to be equally timed spacedfif the pulse 'repetition rate is the same in all trains of the group and if all the pulses of all the trains in the group occur at equal time intervals. Two pulse trains are saidto be coincidentH if the pulses of one train coincide with thej pulses of the othertrain. Combinations of'a plurality of pulse trains all` appearing Vonone` lead such that the'tirne intervals between consecutive pulses appearing `on the lead are not equal are not included within the denition of a pulse train but' are referredto herein as a combination `of pulse trains.

One object of the present invention is to provide a more ecient system for selecting one of a number of circuits calling for connection to an ,exchangeg `Thus, according to`one featureof the present inventionthe selection of a calling circuit is achieved in a number of stages by selecting means whose total number is equal to the number of stages. p ,d

For example, the circuits may be dividedinto groups, selecting means being provided for selecting a group'havingone orif'more callingcircuits and further selecting means being provided for selecting from that group" the one or one of the calling circuits. If the number of circuits isextremely large the time taken to make a selection becomes excessive and can be reduced by further dividing the groups into subgroups so that three nselecting stages areY required. In that case, Vlirsltvselecting means are provided for selecting a group,` second selecting means v areused to vselect a sub-group within the -selected group and third selecting means select a calling circuit within the selected sub-group in the selected group,

In the case of a two stage selecting system the circuits are arranged in a number of groups each of which has an individual indicating lead. Each such lead is associated with means forgenerating a time-spaced pulse train for each circultin the group. A further indicating lead common to `all the groups is `associated with further means for generating a time-spaced pulse train for each group. The time-spaced pulse trains generated by each of the first mentioned means are synchronousand Amay also be synchronous with the pulse trains generated by the further means. Thus, each circuit is characterisedfby thepulse train of the group containing iton the common indicating-lead and the pulse train of the particular circuit on the indicating lead individual to: the Agroup con-` tainingthe. circuit. Firstandsecond selecting means serve the commonindicating lead and thedindividual indicating lead respectively,y the second selecting means lCC being commont'o all the indicating leads individual `to each group.

VEach selecting means preferably includes a plurality `of registering devices such that"tlie number of )combina-A tions of registering devices taken any number at Va time is `at least equal to the maximum number `of pulse trains which may be generated on the associated Vindicating lead. Thus, leach combination of registering devices characterisesa particular pulse train. A pulse train on the indicating lead common to all the groups 'is rst selected and'indicated by a particular combination of registering devices, that indication permitting the pulse trains of calling circuits" in the selected group to be applied to the selecting means Vcommon to all the leads individualto eachgroup' so that` a selection of one circuit can be made. The selected circuit is indicated by the combination of operated registering devices inthe first 4selecting means'together with the combination of `operatedregistering device in the second selecting means.

According to another feature of the 'presentinven'tion in atime division multiplex communication system there `is provided means foncontrolling the setting up ofa connection between a` selected calling circuit and a called circuit in` which the connection is effected -by meansof a register and the connectionbetween the calling circuit and theregister and Vbetween the called circuitV and the register is by common signal circuits and in whichthe pulsetrains used in the said connections may or may not be synchronous.

ThusLin one embodiment, the selected calling `circuit is connected toV a register into which designationV information is transmitted. When the register has received suicient designation information for it to commence setting the connectin it applies to an exchange marker `which when Vfree operates to indicate the required circuit is selected for use in the callcd-circuit-to-register"connection and the setting up of the connection may continue using the twopulse trains.` Alternatively, if the pulse trainhselected for the called-circuit-toeregister connection is found to be free on the calling side of the register it may be substituted for the calling-circuit-to-register` connection and the setting up proceeds using only one pulse train. In this way theV avilable pulse trains are used to the best advantage.

Another feature ofthe present invention is the provision of a system for bridging the connection between the calling circuit and the called circuit when Vthe register has completed the setting up arrangements. `Pulse suppression gates are inserted in the commonsignal circuits and the gates are closed by pulse trains synchronous with the pulse trains used for the calling-circuit-to-register and called-circuit-to-register connections, so `that the `register can sendand receive information over either or both of the circuits without interference. The connection between the calling circuitand the called circuit is not completed until the register is released, the pulse train used for that connection being synchronous with at least one of the pulse trains used for `the calling-circuittoregister and' called-circuitetoLregister connections. As an example a time division multiplex communicatieni-system embodying the various features Vof the present invention will now be described in greater detail with reference to the accompanying drawings of which; Figure 1 is a block schematic showing connections betweenK4 aregister and two groups of circuits, p

Figure'l shows part of Figure 1 in vmore detail,

Figure'3 shows an audio link, and

Figures'4A, 4B, 5A, 5B, 6A, 6B, 7A and 7B when assembled according to Figure 8 represent a block schematic of part of an exchange using time division multiplex switching. t 'Figure `9 shows in block schematic `formdthe essential components of the embodiment. s Y

vReferring first to Figure 9 which lshows in simplified g block schematic form the essential components of the system, there is shown a subscriber SUB A who is one of a group, group 1, of subscribers and second subscriber SUB B of a different group, group Z, of subscribers.

Subscriber SUB A is connected via his line circuit to terminating modulators which, when pulsed Vfrom the pulse store, connect the subscriber in the well known manner of time division multiplexrsystems to'comnion signal circuits comprising Go and Returnf paths individual to the subscribers `forming group 1. Connected vZare connected to the common signal circuits of their groupk in a manner identical with that just described with reference to group 1. The common signal circuits of group Z are also connected in like manner to those of group 1 to the register. Y g Y v,

Although only one register is shown-in` Figure 9, there will normally be a number of these devices all capable of being connected via a register connector to a marker whose function is to mark eacho'f one or more suitable circuits. The registers are also connected to a register/selector which operates to select and mark a free register of the set of registers.

Provided for each group is a group unit which generates for each subscriber in the group either callingfor connection or connected a characteristic pulse train and also an identifying signal in the group when the latter includes calling subscribers. The output of the group unit is applied to selector equipment which selects firstly a group containing one or more calling subscribers and then one of the calling subscribers to which connection can be made to a register marked by the register selector i as free. Y

Whilst the pulse suppression gates referred to above permit connection initially from a calling subscriber to a free register, the gates also prevent connection of the priate moment. It will be appreciated thatlat some stage in the setting up of the connection information is being sent between the calling subscriberand the register and between the called subscriber and the register and it is therefore desirable to prevent transmission between the calling and called subscribers. The manner of effecting this is described in detail below. l

Although in Figure 9, two groups ofsubscribers only calling subscriber to a called subscriber until an approi 4 convert a two wire circuit into a four-wire circuit using a hybrid transformer. The circuits calling forconnection or already connected through the exchange are indicated by the presence of a D.C. signal on leads associated with the circuits. This signal may be derived Y for example by the D.C. changein potential on lone lead of a subscribers pair of leads 'when a loop is completed on the pair of leads by the subscriber calling for connection.

Each circuit is connected to a terminating modulator pair and all the terminating modulator pairs of al group are connectedto a group common lead pair. Thus the transmit channel ATCl of CT1 is connected to group transmit common leadv GT1 via a transmit modulator TM1 when pulsed over pulse distribution leads PDL 1 and Z. TM1, RM1 and DA1 comprise the terminating modulator pair TMPI of circuit CP1. The circuits of GRPZ are similarly connected via terminating modulator pairs to group transmit and receive common leads GT2 and GRZ. The demodulator and amplifiers. associated with a receive modulator enable the audio signal which modulates the pulses received by the receive modulator to be recovered *at substantially -the same level as the original modulating signal. AA hold signal may also be derived .from them which indicates whether or not the receive modulator is receiving-pulses.

Each register in the exchange, one ofwhich REGI is shown in Fig. l is connected via two bothway audio circuits to the exchange transmission means; One both- Way audio circuit referred to as the registerV calling circuit is used for connections between calling circuits and the register and the other bothway audioircuit referred to asY the register called circuit is used'for connections between ca lledtfcircuits and the register. The register calling circuits are connected via register terminating modulator pairs toa calling register calling common lead pair and theregister called circuits are connected via register yterminating modulator pairs to a register called common lead pair. Thus ARCTl the register Acalling circuit of registerREGl is connected via the register terminating modulator pair RMPl comprising transmit modulator RTMI, receive modulator RRMI and demodulator and amplifier RDAI to register calling common lead pair comprising register calling common transmit lead RCGTI and register callingcommon receive lead RCGRl when the register terminating modulator pair RMPI is pulsed over pulse distribution leads PDL 9 and 10. Similarly the register called circuit ARCTZ of REGI is connected via a register terminating group common lead pair is connected to thegroup comn mon lead pair of every other groupl and toY the register are shown it will be understood that there may be more than two groups and that the common signal circuits of each group are interconnected -with the common signal circuits of every other group by vmeans of intermediate modulators.

Circuits which may be subscribers lines, junctions or otherilines external or internal ofthe exchange are ari neet other common lead pairs when pulsed.

ranged in groups, two ot which are shown in Fig. l of the drawingsk and in each group one circuiti's, shown".k Thus circuit CTI is in groupGRPl andcircuit CTZ is in .group GRPZ.4 It will be assumed that thegvfcircuits areA 4-wireat the point where they are connected the exchange. Comunication system` VVV.It is. ,wellrkacrvn .ifo

calling and the register called common lead pairs via pairs of modulators which when pulsed connect the common lead pairs together. Thus intermediate modulator pairy MP1 connects GTI to GRZ and GTZ to GRI when pulsed over leads PDL13` and 15. Intermediate modulator pair MPZ connects GTI to RCGRI and YGRI to RCGTl when pulsed over leads PDLS and 6.l Intermediate modulator pair MP3 connects GTl `to RCDRI and GRI tov RCDTl when pulsed over leads PDL7 and 8, and similarly other` intermediate modulators intercon- 3 byaudiogcircuits., Onesuch pair of terminatingmodulator pairs is 'shown for each group common lead pair inFig. 1.l GTI is connected tolreceive modulator connected to receive modulator RMS which forms a pair, with TM5.a.ud its output is applied via demodulator` and amplier DAS, from which a hold signal is also denived when RM is receiving pulses and the audio out- Y `put of which is connected over audio circuit A2 to transmit modulator TM4, which forms a modulator pair with RM4, and "whose output is connected to GR1. Pulses may berapplied` to RM4 and TM4 over leads PDL13 and 14 and to RMS and TM5 over leads PDL 14 and 15. If pulses are received in Ris/i4 thenthe hold signal derived in DA4 appliedto TM5 causes the pulses applied on; PDL14 and 15 to TM5 to be transmitted to GRI. Similarly if RMS receives pulsesLthe hold signal derived in DAS applied to TM4 causes the pulses applied von PDL13 and 14 to TM4` to Vbe transmitted to GRli. These pulses Yfrom TM5 and TM4 on GRI will be modulated by the audio signals appearing on the audio circuits A1 and A2 respectively. i

Connection between `a calling circuit and a register is made by causing coincident pulse trains to be made effective in the terminating modulator pair ofthe calling circuit, in the register calling terminating modulator pair and in the intermediate modulator pair` connecting the group common` lead pair of the group which includes the calling circuit, to the calling register common lead pair. To set up such a connection so that double connections are avoided, a pulse train which is not in use either in any of the terminating modulator pairs in the calling circuits `group or in any of the group of register calling terminating modulator pairs must be selected for `the connection.

Connection between a called circuit and a register is i made by` causing coincident pulse trains to be made eiec tive `in the terminating modulator pair of the called circuit, the register called` terminating modulator pair and in the intermediate modulator pair connecting the group common lead pairof the called circuits group to the called register commonlead pair. `To setup such a con-Y nection so .that double connections are'avoided, a pulse "train which is notin use either in any of the terminating train which is notin use in any of the other terminating modulator pairs of either the calling or called circuits group must be selected.

Connectionbetween a calling circuit and a called circuit in the same group is made by causing one set of coincident pulse trains to be made effective in the terminating modulator pair of the calling circuit and in the terminating,modulatorvpair at one end `of lan audio link `and a different set ofcoincident pulse trains to be made effective in the terminating modulator pair of thecalled circuit and in the terminating modulator pair at the other end of said` audio link. To avoid undesired double connections neither of these pulse trains must be in use in any of the other terminating modulator pairs in the group. i

` lIt will be clear `to those skilled inthe `art that the physical length of thecommon lead pairs may be such that the time delay in the transmission of a pulse from one end to the other may not be negligible in exchanges inewhichthere aremany groups of circuits. The delay 6 may be allowed for by applying coincident pulse trains to both modulators in each pair of intermediate modulators and by causing the pulse trains applied to the transmit modulators of the` circuits modulator pairs to be phase advanced `such that the time of advance is equal i to the delay of the transmission over the: common transw mit lead. v Similarly the fpulse trains applied to the re-` ceive modulator of the circuits modulator pairs are caused to be phase retarded such that the time by which the pulse' is retarded is equal to the delay of the transmission over the common receive lead. The pulse trains applied to the receive modulators are thus arranged to be a version lof those applied to the transmit modulators delayed by twice the delayof the group common leads. This may be achieved for example by using electrical delay lines in the pulse leads connected to the receive modulators. A

As already described, after a registerI has applied t0- the marker, simultaneous connections exist between the calling circuit and the register, the called circuit and the register and a connectionis prepared but not` cornpleted between Vthe calling and called circuits. In the exchange system herein disclosed the calling and called circuit connection is prepared by causing the pulse train to be used for that connection to be eiective in the appropriate modulators.v Pulse suppression gates (not shown in Fig. 1) are provided in `thegroup common leads and the pulses applied to them over inhibiting leads are Vthose of pulse trains in use on connections between the These points will be `made clearer with reference to Fig. 2. i i

Fig. 2 shows two groups GRPi andGRPZ each having a group common lead pair comprising respectively common transmitleads GT1 and GT2. and common f receive leads GRl and GRZ, the register calling lead pair RCGTI and 'RCGRI and the. register called common lead pair RCDTI and RCDRI. Lead GT1 is connected viapulse suppression gate PSGZ to intermediate modulators,` one of which M1 is shown and which is used to connect GT1 to the common receive lead GRZ. Lead GT 1 is also connected via modulators M3 and M7 to leads RCGRl and RCDRl. Lead GT2 is connected via pulse suppression gate PSG4 to intermediate modulators, one of which M2 forming an intermediate pair with M1, is shown in Fig. `2 and is used to connect GT2 `to GRI. GT2 is also connected to modulators M5 Vand VM9' used to connect GT2 to RCGRI and RCDRI. The

output ofmodulator M1 is connected, in common with the outputs of other modulators used in connecting GRZ to the common transmit leads of `other groups, to pulse `suppression gate PSG3 and thence via decoupling means DMS to GRZ. The output of modulator M2 is connected, in common with the outputs of other'modulators GRPl and GRPZ, respectively. GR11 is connected via decouplingmeansDM to GR1 and is also connected v to the pulse suppression `gates PSGland PSGZ. GRZI is connected via decoupling means DMS: to GR2 and is also connected to pulse, suppression gatesfPSGS land' PSG4. If the calling circuit is ingroup GRPI4 and the called circuit in group GRP2 then provided the pulse ythe connection from the called circuit to the register which appears on GR21 applied to PSG3 and PSG4 is coincident with that used for the calling to called circuit connection, transmission between the two circuits is suppressed in one or other pair of suppression gates.

If the connection is between circuits in the same group then still with reference to Fig. 2, the following pairs of units are identically the same, GT1 and GT2, GR1 and GRZ, DM1 and DMS, PSGI and PSG3, PSGZ and PSG4, GR11 and GR21, M3 and M5, M4 and M6, M7 and' M9, M8 and M10 and connection is made via an audio link as shown in Fig. 3 instead of intermediate modulators as Ml'and M2 of Fig. 2. Fig. 3 shows two terminating modulator pairs TMPI and TMPZ connected by two audio circuits A1 and A2. The connection between calling and ycalled circuits is made using one pulse train eective in the terminating modulator pair of the calling circuit and in TMPl and another pulse train .elTective in the terminating modulator pair of the called circuit and TMPZ. If vthe pulse trains used for either connection to the register, which will appear in Fig. 2 on leads GR11 and GRZI connected to the suppression gates, coincide with either of the pulse trains used for the calling to called circuit connection, transmission between the two circuits is suppressed until the register releases and removes the suppression-pulses from GR11 and GR21. In fact one pulse train would be used for both the calling circuit to audio link and to register connections and another for both the called circuit to audio link and to register connections as will be described later.

In time division multiplex communications systems it is common to transmit over the common leads pulses in the modulator pairs. f This is achieved by inserting a further modulator to which a pulse is applied over a pulse lead in the common lead. The pulse train so applied consists of pulses which lare coincident with, but ofv shorter duration than all the pulses comprising the group of time spaced pulse trains which may be applied to the modulator pairs. The eilects of sofreducing the 'width of the pulses transmitted are that adjacent channel cross talk is reduced and that in the receiver modulator, pulses are gated by longer pulses so that slight delays through the transmission means will not cause any misgating due to slicing olf the back edge of the pulse. Clearly the introduction of this additional modulator may be applied to this invention.

In U.S. Patent No. 2,716,731 granted to T. H. Flowers and I. E. Flood and in U.S. patent application Serial No. 276,306 rin the names of I. E. Flood and S. D. Harper modulator circuits are described which are suitable for use'in embodiments of this invention. It will be clear to those skilled 4in the art thatcommon amplitiers, additional modulators or additional stages of modulation may be inserted at suitable points in the communication systems based on the type described.

In U.S. patent application Serial No. 402,896 in the name of L. R. F. Harris a method is described for distributing pulse trains to groups of units in which a pulse train in a set of pulse trains is required to be made effective in only one unit in the group and in which the generated pulses are effective only in the modulator pair associated with the timing devices producing coincident pulses. The timing devices in this arrangement may, for example, be mercury delay line circulating systems, storage elements, electric delay line circulating systems or other appropriate means. Y

A basic problem in telephone exchange switching sys- -tems is the selection of one, andone only of a number of `Flowers andS; D. Harper and L. R. F. Harris apparatus is described which' may be used for the selection and indication of one circuit out of a group of circuits. Q l

'Alternative selecting techniques ar'e'describedfin 'the U. S. Patent application Serial No. 404,760 in the name of L. R. F. Harris. 'These selectingtech'niques nd application in embodiments of this invention. According to afeature of this invention the selecting devices may be made common to all the groups of circuits `in the eX- change. selection of a particular circuit involves the selection of a group using one selecting device and a subsequent selection of a circuit in that group Vusing a secondselecting device common to allv the groups of circuits' butsed at any one time only for the selected group of. circuits.

In the diagram consisting of -Figs. 4A., 4B, 5A, 5B, 6A, 6B, 7A and 7B arranged as shown in Fig. 8 one circuit CT1 is shown at the top left hand corner of Fig'. 4A and at the point where it is connected't'o the exchange transmission means, comprises a transmit audio pair Vwhich are of shorterV duration than the pulses effective ATCl, a receive audio pair VARCI and hold leads VH1 and H2. Pair ATCI and lead H1 are connected to a transmit modulator TMl. Circuit CT1 is one circuitof a group GRPI and the outputs of all the transmit modulators such as TMl of'the circuits in'the group GRPI are connected to a group transmit common lead`GT1. The group receive common lead GRl of group GRPI is connected to receive modulators associated with each circuit in group GRPl for example to RM1, ythe receive modulator of circuit CT1. The output of modulator RM1 is conected to a receive amplifier, demodulatorand low pass lter DA1, the output of which is connected to pair ARCI. If modulator RM1 is 'transmitting pulses from GRI to DA1 a D.C, hold signal is derived-in rectifier unit RUI from the output of DA1 and this hold signal Vpairs of the circuits of group GRPl.

Group common leads GTl, and GR11, which is connected to GRl via decoupling means DMI, arel connected to register calling common lead pair RCGRl and RCGTl respectively via'a pair of'modulators M3v and M4 respectively, used for connections between calling circuits in group GRPl and the registers. Other group common lead pairs are similarly connected to RCGTl and RCGRI via other pairs of modulators. The modulator pair M3 andv M4 is connected for the purposes of pulse distribution to pulse distribution leads PDLS and PDL6"which form a combination, individual to group GRPl, ,ofV two pulse leads in a set of pulse leads used to distribute pulses to the modulator pairs connecting group common lead pairs to the register calling common lead pair. Group common leads GTi and GRll are also connected to register called common Vlead pair RCDRI and RCDTI respectively via modulators M7 and M8 respectively, used for connections between calledcircutsin group4 GRPl and the registers. Other group common lead pairs are similarly connected to RCDRI and RCDTI via otherl pairs of modulators. The modulator pair M7 and `M8 is connected for the purposes of pulse distribution to pulse distribution leads PDL7 and PDL8 which form a combination, individual to group GRPl of two pulse leads in a set of pulse leads used to distribute pulsfes to the modulator pairs connecting the group conmion lead pairs to the register called common lead pair.

Register calling common lead pair RCGTl and RCGRl and register called common lead pair RCDTI and RCDRI are both connected to the registers via pairs of modulators. Figs. 6A and 6B showfon'e register REGI which is connected to the exchangeltransiiissi Where two stages of selection are'fused the .means ,over` a calling transmit audio pair A3, a calling receive. audio pair A4, ,a calling transmithold lead H3,

calling receive hold `lead H4 and corresponding called leads A5,' A6, H5 and H6. Audio pair A3 and hold flead `H33, are connected to transmit modulator RTMI used to `connect REGI to RCGTl., A5 and H5 are connected to transmit modulator RTMZ used `to connect fREGI to RCDT1. RCGREL is connected to calling receive modulators associated with each register, thus RCGRl isconnected to RRMl, the calling receive modulator of REGl andrRCDl is similarly connected to called receive modulators associated with each register, thus RCDRI is conected to RRMZ, the called receive modulator of` REGi. rhs outputs of RRMi and RRMz are connected to receive amplifiers and demodulators RDAl and RDAZ respectively and thence to A4 Vand A6 respectively. If RRMl or RRMZ is transmitting pulses from RCGRl or RCDRl to RDAl or RDAZ, a D.C. hold signal is derived Vfrom the output of RDAl or RDAZ by rectier units RUZ or RUS so that the hold signal appears on hold lead H4 or H6 respectively.

The modulator pair RTMl and RRMl is connected for `the, purposes of pulse distribution to pulse distribu- `tion leads `lPDL9 and PDL10 which form a combination,

`or some of the hold leads H3, H4, H5 and H6 andthe presence of a hold signal on any of these is used to indicate that REGI is busy.` H3, H4, HS and H6 are connected via suitable decoupling means to register busy lead RBLL Group transmit common lead GTl is connected via `pulse suppression gate PSGZ and decoupling means DMZ to lead GT 12 which forms a pair .with GRIZ. GRYIZ is .connected via pulse suppression gate PSGI and decoupling meansDM1 Vto GR1. Lead GR11 is connected Ato pulsevsuppres'sion gates PSGZ and PSG1 so that pulses are not` transmittedthrough PSGZ and PSGl if they coincide with pulses on GR11. GR11 is connected via limiter LIMl to DMZ so that a coincident unmodulated low amplitude version of the pulse trains on GR11 appears on GTIZ. The output of PSGZ is connected to pulse ,suppression gate PSGS to which pulse hold lead PHLS vis connected so that pulses applied on PHLS to PSGS are suppressed in PSGS if coincident pulses appear on the output of PSGZ. The output OfPSGS is connected via DMZ to GTIZ.

Pairs of leads corresponding to GRiZ and GTlZ are shown in Fig. 6B for two other groups of circuits GRPZ and GRP3. Thus GRZZ and GRSZ of groups GRP?. and GRP3 correspond to GRIZ of group GlPl and GTZZ and GT32 of groups GRPZ and GRPS correspond to GTlZ of group GRPl. The group common lead 4pairs of each group are connected via pairs Vof modulators to the group common lead pairs of every other group.

Thus group common lead pair GT1Z and GR12 are connec-ted to group common lead pair GRZZ and GTZZ via pulse modulator circuits M1 and M2 and to 0R32 and. v

GTSZ via modulator circuits M11 and M12. Similarly GRZZ and GTZZ are connected to GTSZ and GRSZ via pulse modulator circuits M13 and` M14 and other group common lead pairs are lsimilarly interconnected.

Each group commonlead pair is also connected to a number `o`f "pairs of modulator pairs connected together by audio linlts. Fig. 7B shows one such audio link AL1 `for group GRPI comprising two audio pairs A7 and A8,

one for each direction of transmission. "GTlZ is con `through the exchange. 'are associated with timing devices, which may be, for

long as required.

nected` to receive modulators RM4 and RMS `the outputs of which are connected tolampliers, demodulators and low pass filters DA4 and DAS respectively, whose outputs are connected yto audio pairs A7 and Ati and also to RUiand 4RUSrespectively,which generate a `D.C. hold signal on leadsH and H8 `if RM4 or RMS is transmitting pulses from GT12. The audio pair A7 and hold lead H7 are connected to transmit modulator TM5 and A8 and H8 Vare connected to transmit modulator TM4. The outputs of TM4 and TMS are connected to GRlZ. Such audio links are used to connect two circuits in the same group and one modulator pair RM4 and TM4 of audio link A141 would be pulsed by the pulse train used by the calling circuit and the other modulator pair RMS and TM5 by the pulse train used by the calledcircuit. if audio iink ALi is busy a hold signal appears on either or both of the hold leads H7 yand H8, which are connected via suitable decoupling means to the audiolink busy indicating lead ALBl, the potential of which indicates whether or not audio link AL1 is busy. Y

Modulator pairs RM4 and TM4, RMS and TM5, and M1 and M2 are connected for the purposes of pulse distribution to pulse distribution leads PDLiS and PDLM, PDLM and PDLlS, and PDLiS and -PDLIS respectively which form combinations, individual to the pairs of modulators, of two pulse leads each `from a set of pulse leads used to distribute pulses to the modulator pairs of the audio links and to the modulator pairs connecting the group common channels of GRP to some off the other group common channels. it should be noted that the modulator pairs connecting two group common channels may be associated with either group for the purposes of pulse distribution. Thus M1 and MZ are associated with group GRPl but Mii and M12 connecting GRP1 and GRP3 are associated with group GRP3 for the purposes of pulse distribution.

Each `pulse distribution lead in all ot' the sets described is associatedwith a timing device which may be operated to generate on its associated pulse lead any combination of the pulse trains which may be used for connections Thus PDLE., PDLZ PDLlS example, mercury delay line circulating systems, TD1, TDZ TDlS respectively. To cause a particular pulse train to become eiective in a particular modulator pair, it is necessary to cause both the timing devices associated with the modulator` pair to generate that pulse train. Once a timing device has been caused to generate a particular pulse train it must continue to do so for as Themethod of establishing and holding connections through thesystemvwill` now be described with referenceV to Figs. 4A, 4B, 5A, 5B, 6A, 6B, 7A and 7B.

When an external circuit calls, it is first connected to Vdevices distributing pulses to the modulators of the circuits in the group of circuits which includes the calling circuit or'by any of the timing devices distributing pulses to the modulators connecting the calling register common `lead pair to the registers.

The free pulse trains in the two groups must be compared and one of the common free pulse trains must be selected for the connection. i

Register busy lead RBLl is connected to multiplex pulse generator MXl whichgenerates a pulse train which is individual to REGI when REG1 is free. Similarly MX1may generate other pulse trains, one for and indi,- vidual to each register, to indicate other free registers. The pulse trains of all the free registers appear on pulse 'indicating-lead PIL1 which is connected to register selecvtor `SELI which selects one of the free registers and indi Cates the selection on afcombination of D.C. indicating leads individual to the selected register. The combination corresponds to the combination of pulse leads associated with the selected registers modulator pair used to connect it to the register calling common lead pair. Thus if REGI -is the selected register, it is indicated on D.C. indicating leads DCIL9V and DCILI()v corresponding with timing devices TD9 and T1310. As will be described later a calling circuit will be selected and connected to the selected register.

Lead PIL1 is connected to pulse lengthening means PLMl which D.C. indicates on its output register free lead RFLI whether there are any pulses on PIL1 and therefore whether there is a free register. It should be noted that a register which is selected by SELl but not yet connected to a calling circuit, is still indicating its free condition to MXl and its pulse train therefore still appears on lead PILL The outputs of all the timing devices which are used to distribute pulses to the calling modulators of the registers, as for example TD9 and TD10, are connected via suitable decoupling means to pulse suppression lead PSL7 which thus indicatesthose pulse trains unsuitable for use in a new connection between a calling circuit and a register.

Within each group the outputs of all the timing devices used to distribute pulses 'to the circuit modulator pairs, as for example TD1 and TD2 are commoned via suitable decoupling means to a pulse suppression lead, for example PSL2 in group GRPl on which appear pulse trains unsuitable flor use in connections to circuits in group GRPl. PSLZ is connected to pulse suppression gate PSGS() to which all the pulse trains which may at any time be used in connections to GRPI are applied over pulse lead K1. Those pulses in use in the circuit modulator pairs of the group are suppressed in PSG30 and on its output lead PFLI. appear only the free pulse trains suitable for use in a new connection to circuits in group GRPl.

Lead PFLl is connected to pulse gate PCGS to which PSL7 and RFLI are also connected. PCGS and corresponding gates in other groups do not transmit the applied pulses to their output leads if RFLI, which is connected to all such gates, indicates that no register is free. If there is at least one free register, those pulse trains on PFLl which ydo not coincide with those on PSL7 are transmitted to the output lead PILZ of PCGS. Thus, if there are any free registers, the pulse trains suitable for connections between calling circuits in group GRPI and the registers will appear on lead PILZl connected to pulse lengthening means PLM2v which D.C. indicates on its outputy lead whether a calling circuitin group GRPl can be connected to a free register. Corresponding indications are derived in like manner for other groups in the exchange.

ln a group, each circuit is associated with one pulse train, individual to it, in a set of time spaced pulse trains used for pulse indicating the circuits. When a circuit, for example CTl, is ether already connected through the exchange or applying for connection, a hold signal is applied to the exchange. For example if CTI is a subscribers line, the completion of a D.C. loop on the line will cause the appearance of a hold signal on transmit hold lead H1. Lead H1 is connected viaslow to release device SR1 Vtoa multiplex pulse generator MXZ which generates the pulse trains of all those circuits which have a hold signal on their transmit hold leads. The function of. the slow to releasedevice SR1 and the similar devices associated with other circuits is to ensure that the pulse trains generated by MXZ do not disappear during short breaks in the applied hold signal, for example due to dialled impulses. The output of MXZ on lead PL1 is applied to -pulse suppression gate PSG31 in vwhich the lpulse trains of those circuits already selected are Supvlected free register.

o pattern of timing devices.

later. On` the .output lead PFLZ of-PSG31 will appear `the pulse trains of calling circuits in group GRP1` which are waiting to be connected to free registers. PFLZ is connected to pulse lengthening means PLM3 which D.C. indicates whether group GRPl includes a circuit calling for connection to a register. Corresponding indications are derived in like manner for other groups in the exchange.

The outputs of PLMZ and PLM3 are both connected to D C. coincidence gate circuit DCGl which indicates on its output lead DCIL3 the condition that GRPl includes a circuit calling for connection to Va free register and Whether it is possible to make such a connection. There may be more than onegroup in this condition and one of them must be selected as the rst stage in the sclection of one of the circuits calling for connection to the selected register. K

Each group of circuits is associated with one pulse train, in a set of time spaced pulse trains used for pulse indicating the groups. The D.C. output from DCGI is connected by lead DCIL3 to multiplex pulse generator VMX?) as are similar indicating leads for other groups.

MXS generates the pulse trains of those groups in which there is at least one calling circuit connectible to the se- These pulse trains are applied to calling group selecting means SEL2, in which one ofthe groups is selected and indicated on a combination of D.C. indicating leads individual to the selected group. That combination corresponds to the combination of pulse leads associated. with the modulator pair which connects --the common lead pair of the selected grouptov the register calling common lead pair. Thus if GRPl is the. selected group, this fact is indicated on D.C. indicating leads DCILS and DCIL6. The combination of D.C.y indicating leads associated with a group is connected to a coincidence gate associated with said group and thus DCIL5 and DCIL6 are connected -to DCGZ associated with the selected calling circuit. If GRPl is the selected group,

the signal on DCIL4 connected to gate PG16 causes the pulse trains of the calling circuits in the selected group, applied from MX2 on PFLZ to be transmitted to the common circuit pulse indicating lead PIL3. PILS is connected to circuit selecting means SEL3 which selects a. calling circuit in the selected group.. The selected calling circuit is D.C. indicated on a combination of D.'C. indicating leads corresponding to the combination of pulse leads used for distributing pulse trains to the modulator pair of the selected circuit. It will be appreciated that SEL3 indicates the combination of pulse leadsto be used Y but does not identify the circuit since the same combination of leadsmay be associated with different circuits Vin diiferent groups and to identify a circuit it is also necessary to indicate the group. It will be clear that circuits in different groups are associated for the purposes of pulse iudication with the coincident pulse trains and each such circuit will be associated with the same combination or If the selected pattern corresponds to that of CTI in group GRPl, this fact is indicated on leads DClLl and DCILZ connected.y to timing devices in all the groups of circuits. The selected pattern is also pulse indicated by its associated pulse train on lead PIL4 and this will coincide with the pulse train of the selected circuit.Y `Lea iPIL4 is` connected to pulse gates associated with each group and the one associated suppression lead PSLS whose operation will be described with selected group transmits the applied pulse train. Thus if GRPl is'the selected group, the indication on DCIL4, generated by SELZ when GRPI is selected, allows -.'PG19 totransmit the pulses `TD2.0.which.becomes operated to generate the pulse `PHLl connected to pulse coincidence gate PCGZ which'` may be `in the recirculation path of a mercury delay. line circulating system if that is the apparatus used Vfor TDZ. PHLl `is connected via 'decoupling means DMZtl` to lead PL1 which is the output lead of MX2 already described. PHL1 is also connected via DM20 to lead PL2, the output `of multiplex pulse generator MX4 which generates the pulse trains `of those circuits in the group, held from within the exchange. Thus if a hold signal appears on H2, MX4 generates the pulse train of circuit CTI on lead PL2. Slow to release devices, as SR2, are interposed between hold leads, as H2, and MX4 and they ensure that the .pulse trains generated do not disappear during short breaks in the `applied hold, due to accidental breaks for example. So` long as a hold signal is present on either hold lead of a, circuit, its associated pulse train appears on PHLI and once the circuit has been selected its pulse train will continue to appear on PSLS until both holdsignals disappear.

"Lead DCIL4 which indicates whether GRPI is the selected group, is also connected to gate PCGSS which, when GRPI is the selected groupVtransmits the pulse trains suitable for connecting the selected calling circuit to the selected register from vlead PILZ over corn-mon pulse indicating lead PILSl to pulse train selecting means SEL4. SEL4 selects one of said suitablepulse trains and pulse indicates the selected pulse trainon lead PIL6.`

f It isnow necessary to operate Ythe timing devices associated with the selected circuits modulator pair,as sociated with the modulator pair on the calling side of `the selected register and associated with the modulator ,14 set of timing devices used to distribute pulses to themodulators of the groups circuits. The combination` ofthese gates which is D.C. indicated by SEL3 transmits ,the selected pulse train to their associatedr timing devices which thengenerate the selected pulse train which is thusmadle elective in the modulator pair of the selected calling circuit. Thus if circuit CTl in group GRPI is the selected Y `calling circuit, the D.C. indication on DCIL4 opens gate pair used to connect the selected groups common lead Apulse train thus causing it to be effective in the modulator pair on the calling side of the selected register. Thus if REGI Vis selected, the D.C. indication on `DCIL9 and DCIL1() `opens gates PG9 and PGI@ which transmit the selected pulse train applied to them on PIL6 to timing de- Vvices`TD9 and TD1() causing them togeneratethat pulse train on leads PDL9 and PDLIG.

, `Lead PIL6 is also Vconnected to gates associated with each timing device kin the set of timing devices used Vto distribute pulse trains to the modulator pairs connecting .group 'common leadpairs to the register calling common lead pair and the combination of these, which is .D.C. indicated bySELZ transmit the selected pulse train to the` associated timing devices which become operated `to generate that pulse train, which thus becomes etec- Y tive inthe modulator pair connecting the selected groups common lead pair to the register calling common lead pair. Thus if GRPI is the selected group, the D.C. indication on `D ClL and DCIL6 opens gates PGS and PG6 which transmit the selected pulse train applied onlead PIL6 to timing devices TD5 and TD6 causing themV to generate said pulseftrain on leads PDLS and PDL6.

lLead PIL6 is also connected to gates associated with each group to which the D.C. indicating lead, as DCIL4 v,is connected. `In the selected group the D.C. indication thereon opens its gate to transmit the pulse train on ,lead PIL6 to gates associated with each timing device in the PGiwhich transmits the selected pulse train appliedon PIL6 to PL3-and lthe D.C. indication on DCIL1'and DCILZ opens gates PGI and PGZ which transmit the selected pulse train applied on PLS to timing devices TD1 and TD2 causingsaid pulse train 'to appear on pulse leads `PDLl and PDLZ.

The presence of the hold signal on the hold `lead con nected to the transmit modulator of lthe selectedcalling circuit then causes the selected pulsetrain Vto appear on the common transmit lead of the selected group. This will be transmitted through the modulator connecting said common lead to RCGRI and thence through the receive modulator RRMl on the calling side of the selected register. After a fewpulses a holdrsignal Vwill appearA on the calling receive hold lead, such as H4, `and the register willV indicate its busy condition on its busy indicating lead, such as RBLl. The receiptof sucha hold signal can, by known techniques, initiate a transmit hold signal on the calling` transmit hold lead, such as H3, causing the selected pulse train to appear on RCGTI. This `will be transmitted through the modulator connecting RCGTl to thecommon receive lead of the selected group and thence through the receive modulator such as RM1 of the selected calling circuit Where after a few pulses, a hold signal will appear on 4the receive hold lead, such as H2, and this will cause the pulse train of the selected circuit to be generated by the multiplex, as MX4. If theselectedcircuit is in group GRPl, Vthe pulse train individual to the selected circuit will thus appear on lead PL2. This lead is connected to a gate PG20` which has been yopened by the D.C.indication on DCIL4 `if GRP1 is the selected group. The gate PG20 then transmits the pulse trains on lead PL2 to common pulse indicatingQlead PIL7. Those pulses are compared in pulse coincidence gate PCG4 with those` on lead PIL4 which' is pulse indicating the selected circuit. As` soon as the pulse train on PIL4 coincides with one on PIL7 it is known that all the selecting and `setting up operations have been completed and that there is `at least'pulse transmission in both directions between the calling circuit andthe register. The output of PCG4 is used to release the selecting means SELL SELZ, SELS and SEL4 by the use of techniques described in the specification of U.S. Patent No. 2,721,094 granted to T. H. Flowers, S.l D. Harper and LQ R. F. Harris.

The presence of a transmit hold signal on the calling side of the registerfcauses the pulse train used in the registers connection to a calling circuit to appear on RCGTI and this is connected to the timing devices in the sets of `timing `devices used to distribute pulse trains to the register calling modulator pairs and to the intermediate modulator pairs connecting the group common lead pairs to the register'calling `common lead pair. RCGTI is connected to said timing devices via pulse suppression gate PSG3 in which a pulse train may be sup pressed by the application of a coincident pulse train on lead PL4. The output ofPSGS` is used to maintain the storage ofthe pulse trains in the appropria-tetiming devices. This may be achieved by applyingthe pulses on maintaining Vlead PHLZ, the outputlead of PSG3 to coincidence gates in the circulation paths of mercury delay line circulating system timing devices.` For example PHLZ is connected to gatesPCGS, PCGd, PCG9 and PCGltassociated `with. timing devices TD5, TD6, 4'I`D9 and TD1() respectively and if a connection is between a calling circuit in group GRPI and REGI,` the pulse train .this would happen will be described later.

In a connection between a calling circuit in group GRP and REG1 the presence of the hold signal on H3 also causes the pulse train used to appear on leads GRll and GRI. The presence of the pulse trainen GRll, connected to pulse suppression gates PSGl and PSGZ prevents the transmissionof the pulse train through them. GRll is also connected to limiter LIMl and a low amplitude version of the pulse train will appear on GT12. The presence of the pulse train on GRI connected to pulse maintain lead PI-IL3 is used to maintain the storage lof the pulse train in the appropriate timing devices in the set of timing devices used to distribute pulse trains to the calling modulator pairs of GRPI.V Thus PHL3 is connected to PCGland PCGZ, associated with timing devices TD1 and TD2 respectively and if a pulse train appears on lead PHL3 which coincides with one on the outputs of the yconnected to the called circuit.

Once ya calling circuit has become connected to a register, the register will receive designation information coded, for example, as dialled pulses or pulses of V.F. signals, etc. YThis information may be stored in the register using known techniques. When sucient information is stored in a register for it to start setting up the connection the register is connected via a suitable register connector RC to the common marker MKRI which receives v the designation information `stored in the register and indicates the circuits, one of which is required to be connected to the calling circuit connected to the register which is connected to the marker, hereafter referred to as the* callingcircuit and the register respectively The marker'` indicates circuits suitable for the connection by D.C. indication on a lead common to them. For private subscribers lines, usually only one circuit will be marked,

butif a P.B.-X group is indicated, all the circuits in the group are indicated by a common indicating lead and similarly for junctions to another exchange. It is assumed herein that these functions of a register and marker are `known and that they are performed by known techniques.

As has already beendescribed it is necessary to be able to connect a called circuit to the same register as the calling circuit and also to prepare the connection between the calling and called circuits. To make theiirst connection it is necessary to find a pulse train which is not in use in the modulator pairs of a group which includes a free called circuit and also not in use in any of the modulator pairs on Ithe called side of the registers. lf such a pulse train can be found, a connection between a called circuit and the register can be made. To make a connection between two circuits in derent groups it is also necessary to find a pulse tram which is not in use in the group of modulator pairs of a group of circuits in which there is a free called circuit which can be connected to the register and which is also either notin use in the modulator pairs of the calling circuits group, or which is already in use in the connection of the calling circuit to the register. For such a connection between vcircuits in dilerent groups it is necessary to ensure that the pulse train used for the calling tto 'called circuit connection coincides with either or both of lthose usedgfor the connections between them and the tregl-ster. 1

' These requirements are met if the pulse train used l for the vcalling circuit to -used for the called circuit to register'connection.

. circuit connection.

register connection vis not in use in the modulator pairson the called side of the registers or in the modulators of a free called circuits groups modulator pairs. All three connections then use the same pulse train. They are also metif a pulse train can be found -free inthe modulator pairs of the calling circuits group, a, free called circuits group and the registers calling and called groups. Again all three connections may yuse the same pulse train but it is necessary to change the pulse train used in the calling circuit to register connection when setting up the other connections. Therequireinents are also met if a pulse train can be found free in the modulator pairs of the calling circuits group and another pulse Vtrain found free in the called circuits group and the registers called group. The calling circuit to register then continues to use the same pulse train and a second pulse train is used for the other two connections. The requirements are also met if the pulse train used in the calling circuit to register connection is free in the modulator pairs of a free called circuits group from which a connection can be made to the called side of the register and also free in the modulator pairs of the called side of the registers. The calling circuit to register connection then continues to use the same pulse train which is also used for the calling to called circuit connection and a second pulse train is The requirements are also met if a pulse train can be found which is free in the modulators of the calling circuits group, of the `registers calling group and of a freel called circuits group whose circuits .can be connected to the registers called side. The calling circuit toregister connection is changed during the setting up of the connection and then uses the same pulse train as the calling'to called Another pulse train is used for `the called circuit to register'connection. Any of these patterns of pulse train arrangement meet the requirements and which pattern `is used depends upon the pulse trains which are free in the various groups when the connection is set up. y

If the connection between two circuits in different groups cannot be made because a free pulse train cannot be found, `but there is a free called circuit in the calling circuits group which can be connected to the register, then connection between the calling circuit to called cirvcuit is made viavan audio link. The calling toregister pulse train is also used to connect the calling circuit to the audio link and the called circuit to register pulse train is also used to connect the called circuit to the audio link.

To set up suchk connections it is necessary to know lthe pulse trains free in the various groups and to identify` the calling circuit and group. There are Various ways in which the suitable pulse trains, groups and circuits may to lD.C. and pulse pulse coincidence gate DPCGl lto which PDL9 and PDL10 are also connected. This D.C. potential causes the pulse train appearingcoincidently on PDL9 and PDL10 to be transmitted to PIL8. PIL8 `issimilarly connected to gates associated one with each register and to each such gate the associated combination of pulse leads is connected.

PILS is also connected to gates, one for each group of circuits, towhich are connected the combinations of pulse registers -leads connected to the modulator pairs which connectthe common lead pairs to the register calling common lead pair. The pulse train is only transmitted through the gate associated with the group which includes the calling whether GRPI includes the calling circuit, to which a g free called circuit is to be connected. DCL4 is connected to pulse gate PGS to which the pulse trains, not already in use in GRPI .are applied en lead PFLl. If GRPI in i cludes the calling circuit, the D.C. signal on DCL4 opens PGS which transmits said free pulse `trains on PFLI to pulse free indicating lead PFL3. PIL9 is also connected to pulse coincidence gates associated with each of the timing devices in the group GRPI, used to distribute pulses tothe groups modulator pairs. In these pulse coincidence gates the pulse trains stored in the timing devices are compared with the pulse train on lead PIL9. The pulse coincidence gates of the timing devices forming the combination of the calling circuit will transmit the pulse train and if the outputs from these pulse coincidence gates are applied to pulse lengthening means, the calling circuit is D.C. indicated conveniently by the signals on the outputs of the said pulse lengthening means.` Thus in group GRPl, PlL9 is connected to PCGlG and PCG17 which are associated with timingdevices TD1 and TD2 respectively and to which PDLl and PDLZ are connected. If CTl is the calling circuit, the pulse train on 1:IL9` coincides with the pulse train which is stored in both TD1 and TD2 and the pulse train on PlL9 is transmitted through both PCGl-S and PCGl'. The outputs of PCG16 and PCG17 are connected to pulse lengthening means PLM4and PLM respectively, whose outputs D C. indicate that the calling circuit is CTL or more exactly, that the timing devices TD1 and TD2 are the timing devices in group GRPl which are associated with the calling circuit.

Thus the calling circuit, the calling circuits group, the pulse train used for theconnection between the calling circuit and the register and the free pulse trains `in the calling circuits group are all indicated when `the `register' is connected to the marker. To change the pulse train used for the calling circuit to register connection,` as'` is sometimes required, it is necesary to indicate the timing devices associated with the register and with `the modulator pair connecting the common lead pair of the calling circuits group to the register calling common lead pair. This is done by comparing the pulse train on PILS with the pulse trains generated by the timing` devices used to distribute pulses to the modulator pairs on the calling side of the registers and to the modulator pairs connecting group common lead pairs to the register calling common lead pair, Thus lead 1311.8 is connected to "pulse coincidence gates associated one with each timing device in the two sets of timing devices, for example, PCGZS, PCG26, PCG29 and PCGS() associated with TD5, TD6, TD) and TD1@ respectively leads PDLS, PDL6, PDL9 and PDLlil are connected to PCGZS, PCGZlS, PCG29 and PCGS respectively and if the pulse on PILS is transmitted through these four pulse coincidence gates it is known that group GRPI is the calling circuits group and that REGI is the register. This is conveniently indicated by the outputs from pulse lengthening means to which the outputs of the pulse coincidence gates are connected. Thus the outputs of PCGZS, PCG26, PCGZ9 and PCG30 are connected to pulse lengthening means PLMS, PLM6, PLM9 and PLM10 respectively and the D.C. outputs of these and the similar units associated with other timing devices in the sets indicate those timing devices in which the pulse train may have to be changed.

The set of output leads from the pulse lengthening means associated one with each timing device in lthe set used to distribute pulses to the modulator pairs on the calling side of the registers may also be used to indicate` directly the timing devices to be used to distribute pulses` to the modulator pair on the called side ofthe` register.

. 18 The combinations associated with the registers in the two sets of timing devices then correspond with each other. If REGl is the register connected to the marker, the timing devices TD@ and TD1@ are those used in the connection to the calling circuit and the outputs of PLM@ Iand PLMltl on leads DCILll and DCILllZ indicate this and they will also indicate that TDH and TD1?. are the timingdevices to be used in the called circuit to register connection. .Similarly other combinations: of timing devices will be indicated when other registers are connected to the marker.

The outputs of all the timing'devices distributing pulses tothe modulator pairs on the called side of the registers are connected viarsuitable decoupling means to a common pulse suppression lead PSLS. yOn this lead appear all those pulse trains which are unsuitable for the connection between a free called circuit and the register.

As already described, those pulses not used to dis* tribute pulse trains to the modulator pairs of a group appear on pulse free indicating leads such as PFLl in group G'RPI. Lead PFLl is connected to pulse suppression gate PSG4 to which PSL6 is connected and the only pulse trains transmitted through PSG4 are those which are free, both in the group of modulator pairs of GRPI and in the modulator pairs of the registersV called side. If any pulse trains are so transmitted it means that called circuits `in the group can be connected to the registers. The output of PSG4 is pulse lengthened in lLMS which is connected to pulse gate P624 to which PFLI is also connected. Pulse trains on PFLll are transmitted through PG24 tofPL`6 only if the output from PLMS indicates that connection between a called circuit in the group can be made to` a register. `Corresponding leads to PLG in other groups also indicate the pulse trains suitable for connection to called circuits in the groups, provided connection can be made to the register.

It has been shown so far how the following information may be indicated: the calling circuits timing devices; the -calling circuits group, the free pulse trains in the calling" circuits group, the pulse train already used for the calling circuit to register connection, the timing devices used in connecting the calling circuits group to the register calling common lead pair, the timing devices on the calling side of the register, the timing devices on the called side of the register, the pulse trains already in use in the group of register modulator pairs on the calling side, the pulse trains already in use in the group of register modulator pairs on the called side, the groups of circuits in which it is possible to connect a called circuit to a register and the pulse trains which are free in such a groups modulator pairs. It remains to be shown howallfthis information can be used to set up the required connections when a circuit or a number of circuits in one or several groups is indicated as being a suitable outlet for the connection through the exchange. It is convenient to assume that circuit CT 1 in group GRPI is among those suitable external circuits.

The marker MKRI D.C. indicates the suitable circuits and a signal on marking lead MLl is used to indicate that CT 1 is among the suitable called circuits. MLl is connected to multiplex pulse generator MXS which when an indicating signal appears on leads suchv as MLl, causes the individual pulse trains ofV the D C. indicated circuits Vto be generated on its output lead PLS. These pulse trains coincide in time with those generated by MXZ and IMX-4 when the same circuits are indicated by those multiplex pulse generators. Pulse lead -PLS is con'- nected to pulse suppression gate PSG59 to which PFLS,

p on which appears the individual pulse trains of al1 the atleast one free'called circuit. Corresponding leadsin 

